U.S. patent number 10,018,430 [Application Number 14/210,616] was granted by the patent office on 2018-07-10 for heat transfer system and method incorporating tapered flow field.
This patent grant is currently assigned to ROCHESTER INSTITUTE OF TECHNOLOGY. The grantee listed for this patent is Satish G. Kandlikar. Invention is credited to Satish G. Kandlikar.
United States Patent |
10,018,430 |
Kandlikar |
July 10, 2018 |
Heat transfer system and method incorporating tapered flow
field
Abstract
A heat transfer system including a fluid inlet; a fluid outlet;
and a substrate in fluid communication with the fluid inlet and
fluid outlet, the substrate including a heat exchange region having
a heat transfer surface and a flow field adjacent the heat transfer
surface, the flow field including a fluid flow area including an
open region at the inlet, a heat transfer region in thermal
communication with the heat exchange region, and a taper of the
flow field cross-sectional area in the flow direction, wherein the
flow field heat transfer region includes a plurality of spaced
apart open enhancement features from 1 micron to 3 mm in size, and
method for enhancing the heat transfer performance of an apparatus
is disclosed.
Inventors: |
Kandlikar; Satish G.
(Rochester, NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kandlikar; Satish G. |
Rochester |
NY |
US |
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Assignee: |
ROCHESTER INSTITUTE OF
TECHNOLOGY (Rochester, NY)
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Family
ID: |
51522233 |
Appl.
No.: |
14/210,616 |
Filed: |
March 14, 2014 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140262186 A1 |
Sep 18, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61782458 |
Mar 14, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28F
13/08 (20130101); H01L 23/473 (20130101); H01L
2924/0002 (20130101); F28D 2021/0028 (20130101); F28F
3/02 (20130101); H01L 23/427 (20130101); H01L
23/3672 (20130101); H01L 2924/0002 (20130101); H01L
2924/00 (20130101) |
Current International
Class: |
F25D
15/00 (20060101); F28F 13/08 (20060101); H01L
23/473 (20060101); H01L 23/427 (20060101); F28F
3/02 (20060101); F28D 21/00 (20060101); H01L
23/367 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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08271185 |
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Oct 1996 |
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JP |
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2013066271 |
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May 2013 |
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WO |
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Other References
JP_08_271185_A as cited on the IDS description translation. cited
by examiner .
The effect of inlet constriction on bubble growth during flow
boiling in microchannels; A. Mukherjee a,, S.G. Kandlikar b;
International Journal of Heat and Mass Transfer; accessed Oct. 19,
2016 :
http://www.rit.edu/.about.w-taleme/Papers/Journal%20Papers/J083.pdf.
cited by examiner .
Numerical Study of the Effect of Inlet Constriction on Bubble
Growth During Flow Boiling in Microchannels ; Abhijit Mukherjee ,
Satish G. Kandlikar, Proceedings of ICMM2005 3rd International
Conference on Microchannels and Minichannels ; Jun. 13-15, 2005,
Toronto, Ontario, Canada ; accessed Oct. 19, 2016
http://www.ritedu/.about.w-taleme/Papers/Conference%20. cited by
examiner .
International Search Report and Written Opinion of corresponding
application No. PCT/US2014/027246 dated Aug. 25, 2014. cited by
applicant .
Balasubramanian, Lee, Teo and Chou, Flow Boiling Heat Transfer and
Pressure Drop in Stepped Fin Microchannels, International Journal
of Heat and Mass Transfer, vol. 67 (2013) pp. 234-252. cited by
applicant .
Miner, Phelan, Odom and Ortiz, Experimental Measurements of
Critical Heat Flux in Expanding Microchannel Arrays, Journal of
Heat Transfer (Oct. 2013) vol. 135, Issue 10, 8 pages. cited by
applicant .
Kandlikar, Satish G., Widger, Theodore, Kalani, Ankit and Mejia,
Valentina, Enhanced Flow Boiling Over Open Microchannels with
Uniform and Tapered Gap Manifolds, Journal of Heat Transfer, vol.
135 (Jun. 2013) pp. 1-9. cited by applicant .
Tanda, Giovanni, Heat Transfer in Rectangular Channels with
Transverse and V-Shaped Broken Ribs, International Journal of Heat
and Mass Transfer, vol. 47 (2004) pp. 229-243. cited by applicant
.
Liu, Y., Cui, J., Jiang, Y.X., Li, W.Z., A Numerical Study on Heat
Transfer Performance of Microchannels with Different Surface
Microstructures, Applied Thermal Engineering, vol. 31 (2011) pp.
921-931. cited by applicant .
Zhigang, Liu, Ning, Guan, and Chengwu, Zhang, Influence of Tip
Clearance on Heat Transfer Efficiency in Micro-Cylinders-Group Heat
Sink, Experimental Thermal and Fluid Science (2012)
http://dx.doi.org/10.1016/i.expthermflusci.2012.11.021. cited by
applicant .
Tullius, J.F., Tullius, T.K., and Bayazitoglu, Y., Optimization of
Short Micro Pin Fins in Minichannels, International Journal of Heat
and Mass Transfer, vol. 55 (2012), pp. 3921-3932. cited by
applicant .
Reyes, M., Arias, J.R.,Velazquez, A., and Vega, J.M., Experimental
Study of Heat Transfer and Pressure Drop in Micro-Channel Based
Heat Sinks with Tip Clearance, Applied Thermal Engineering, vol. 31
(2011) pp. 887-893. cited by applicant .
Supplementary European Search Report in correspondence European
Application No. EP14767495 dated Sep. 15, 2016. cited by
applicant.
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Primary Examiner: Jones; Gordon
Attorney, Agent or Firm: Bond, Schoeneck & King, PLLC
Noto; Joseph
Parent Case Text
CROSS REFERENCE
This application claims the benefit of U.S. Provisional Patent
Application Ser. No. 61/782,458, filed Mar. 14, 2013, the contents
of which are hereby incorporated by reference in its entirety.
Claims
What is claimed is:
1. A method for enhancing the flow boiling heat transfer
performance of an apparatus, comprising: providing a heat transfer
system comprising: a fluid inlet; a fluid outlet; a substrate
through which heat is transferred across, a portion of the
substrate surface comprising a heat transfer region, which
comprises a heat transfer surface having a plurality of enhancement
features from about 1 micron to about 3 mm in height on the surface
of the substrate; and a flow field comprising an open region
extending between the fluid inlet and fluid outlet and a fluidic
region comprising the area between the enhancement features, the
fluidic region in fluid communication with and adjacent to the open
region, wherein in any cross-section of the heat transfer region
fluid in the heat transfer region is in fluid communication with
the open region and wherein the flow field cross-sectional area
increases in the fluid flow direction by increasing the height
normal to the substrate of the flow field and the open region;
flowing fluid in the fluid inlet through the flow field; nucleating
bubbles in the fluid within the heat transfer region, wherein a
plurality of bubbles emerge into the open region and liquid refills
the space vacated by the bubbles over the heat transfer surfaces
causing the heat transfer surfaces to rewet; flowing liquid and
vapor out the fluid outlet in a manner to transfer heat through the
substrate to the flow field which reduces the acceleration pressure
drop as well as total pressure drop and enhances the critical heat
flux and heat transfer coefficient during flow boiling.
2. The method of claim 1, wherein the enhancement features comprise
at least one of micro fins and microchannels.
3. The method of claim 1, wherein the plurality of enhancement
features is comprised of a plurality of open channels.
4. The method of claim 1, wherein a taper is comprised of varying
the density of the plurality of enhancement features.
5. The method of claim 1, wherein a taper is comprised of varying
the geometry of the plurality of enhancement features.
6. The method of claim 1, wherein a taper comprises a ratio of the
maximum fluid flow cross-sectional area to the minimum fluid flow
cross-sectional area in a range of from about 1.0001 to about
1000.
7. The method of claim 6, wherein the range is from about 1.001 to
about 100.
Description
FIELD
The present invention relates to a heat transfer system and method
incorporating a tapered flow field, and in particular to a heat
transfer system and method incorporating a taper in the
cross-sectional area of the flow field.
BACKGROUND
For the last several decades, air has been the preferred fluid for
cooling electronics due to its availability, low cost (cooling
fans), and reliable system operation. As chip power densities
increased, liquid cooling systems have been introduced. Pioneering
work revealed the potential for electronics cooling with
microchannels. Liquid cooling in microchannels can be used to
dissipate heat fluxes of approximately 1 kW/cm.sup.2. However,
temperature variation of the chip surface along the coolant stream
and large pumping power required for this system are of concern.
Flow boiling in microchannels was expected to provide effective
cooling without these concerns; however, current research indicates
that such high heat fluxes are not projected with current
microchannel designs.
Heat transfer in a microchannel or a minichannel is efficient
because of the small hydraulic diameters in these channels. Typical
channel hydraulic diameters range from 200 micrometers to 3 mm for
minichannels and below 200 micrometers for microchannels. The small
hydraulic diameters also give rise to a high pressure drop. The
increased pressure drop leads to higher pumping power, increased
fluid pressure, and a steep pressure gradient in the channels along
the flow direction.
Enhancing the channels with surface features, such as roughness,
corrugations, turbulators, flow disruptors and fins provide further
enhancement in heat transfer, but these features also increase the
pressure drop.
For both single-phase flow and flow boiling applications, there is
a need for heat transfer enhancement strategies that do not result
in significant increase in the pressure drop. Alternatively, there
is a need for heat transfer enhancement strategies that increase
heat transfer for a given pressure drop or a given equipment size
in single-phase flow and two-phase flow including flow boiling and
flow condensation. In the case of a two-phase flow, there is
additional need to provide a stable flow. In the case of flow
boiling, high critical heat flux (CHF), high heat transfer
coefficient, and low pressure drop are desired.
SUMMARY
In accordance with one aspect of the present invention, there is
provided a heat transfer system including a fluid inlet; a fluid
outlet; and a substrate in fluid communication with the fluid inlet
and fluid outlet, the substrate includes a heat exchange region
having a heat transfer surface and a flow field adjacent the heat
transfer surface, the flow field includes a fluid flow area
including an open region at the inlet, a heat transfer region in
thermal communication with the heat exchange region, and a taper of
the flow field cross-sectional area in the fluid flow direction,
wherein the flow field heat transfer region includes a plurality of
open enhancement features from about 1 micron to about 3 mm in
height.
In accordance with another aspect of the present invention, there
is provided a method for enhancing the heat transfer performance of
an apparatus, including providing a heat transfer system including
a fluid inlet; a fluid outlet; and a substrate in fluid
communication with the fluid inlet and fluid outlet, the substrate
includes a heat exchange region having a heat transfer surface and
a flow field adjacent the heat transfer surface, the flow field
includes a fluid flow area including an open region at the inlet, a
heat transfer region in thermal communication with the heat
exchange region, and a taper of the flow field cross-sectional area
in the fluid flow direction, wherein the flow field heat transfer
region includes a plurality of open enhancement features from about
1 micron to about 3 mm in height; and flowing fluid to the fluid
inlet, through the flow field and out the fluid outlet in a manner
to transfer heat in the heat exchange region.
These and other aspects of the present invention will become
apparent upon a review of the following detailed description and
the claims appended thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an embodiment showing a schematic cross-sectional view of
the assembly along the length of a plurality of open microchannels
in a flow field with negative taper having decreasing gap in the
flow direction;
FIG. 2 is an embodiment showing a schematic cross-sectional view of
the assembly along the length of a plurality of open microchannels
in a flow field with positive taper having increasing gap in the
flow direction;
FIG. 3 is a schematic showing an embodiment of positive taper;
FIG. 4 is a schematic representation of flow cross-sectional area
in the heat transfer region and the open region of the flow
field;
FIGS. 5A and 5B are schematics of an embodiment showing a radially
expanding flow with a radial tapered gap in the cover;
FIGS. 6A and 6B are schematics of an embodiment showing a
rectangular fin region and cover with tapered gap and having
rectangular fluid inlet and side outlet ports;
FIG. 7 is a schematic of the open microchannels with tapered gaps
for stable, enhanced flow boiling;
FIG. 8 is a schematic of the test section and the heater assembly
used in the examples;
FIG. 9 is a schematic of a 3 mm thick copper chip with 217 .mu.m
wide, 162 .mu.m deep, and 160 .mu.m fin width in the central 10
mm.times.10 mm region (left image) and 2 mm wide.times.2 mm deep
groove on the underside (right image);
FIG. 10 is a graph of the heat transfer performance of Experiments
A1 (V=40 mL/min, S=0.127 mm), A2 (V=225 mL/min, S=0.127 mm), B1
(V=40 mL/min, S=0.254 mm), B2 (V=225 mL/min, S=0.254 mm) with heat
flux plotted vs. wall superheat described in Example 1;
FIG. 11 is a graph of the flow boiling performance of the
microchannel chip with uniform and the three tapered gaps C1, C2,
and C3 in Example 2;
FIG. 12 is a graph of the pressure drop performance of microchannel
chip with uniform gap U2 and tapered gaps C1, C2 and C3 in Example
2;
FIG. 13 is a graph of the flow boiling performance of the
microchannel chip with a tapered gap D described in Example 3 with
a maximum heat flux (CHF) of 648.5 W/cm2 at a wall superheat of
13.04.degree. C. with a record heat transfer coefficient of 497,300
W/m2.degree. C.; and
FIG. 14 is a graph of the pressure drop performance of the
microchannel chip with tapered gap D described in Example 3 with a
pressure drop of 6.41 kPa at the maximum heat flux of 648.5
W/cm2.
DETAILED DESCRIPTION
The present invention provides for overall enhanced heat transfer
performance, preferably, for example, a significant reduction in
the pressure drop at the same, higher or slightly lower heat
transfer as compared to conventional macro and micro scale heat
transfer systems and apparatus. The term fluid includes
single-phase fluid (liquid or vapor) and two-phase fluid (liquid
and vapor). The term vapor includes gas. The fluid substance in
accordance with the present invention includes water, refrigerants,
chemicals, petrochemicals, mixtures of two or more substances, air,
cryogenic substances, and the like. The invention functions to
provide an efficient heat transfer configuration to dissipate
higher heat fluxes at lower wall temperatures, and/or increase
critical heat flux in the case of flow boiling. In the case of
single-phase flow, this invention serves to provide an efficient
heat transfer configuration to transfer higher heat fluxes for a
given pressure drop. The lower pressure drop in one embodiment also
enables increasing the length of the heat exchanger thereby
increasing the heat transfer surface area and the heat transfer
rate. The invention can be used to reduce temperature
non-uniformity by controlling the heat transfer coefficient and/or
heat transfer surface area variation along the flow direction.
In accordance with an embodiment of the present invention, a heat
transfer system includes a fluid inlet; a fluid outlet; and a
substrate in fluid communication with the fluid inlet and fluid
outlet, the substrate includes a heat exchange region having a heat
transfer surface and a flow field adjacent the heat transfer
surface, the flow field includes a fluid flow area including an
open region at the inlet, a heat transfer region in thermal
communication with the heat exchange region, and a taper of the
flow field cross-sectional area in the fluid flow direction,
wherein the flow field heat transfer region includes a plurality of
open enhancement features from about 1 micron to about 3 mm in
height, preferably from about 10 microns to about 1 mm in
height.
In accordance with another embodiment of the present invention, a
method for enhancing the heat transfer performance of an apparatus
includes providing a heat transfer system including a fluid inlet;
a fluid outlet; and a substrate in fluid communication with the
fluid inlet and fluid outlet, the substrate includes a heat
exchange region having a heat transfer surface and a flow field
adjacent the heat transfer surface, the flow field includes a fluid
flow area including an open region at the inlet, a heat transfer
region in thermal communication with the heat exchange region, and
a taper of the flow field cross-sectional area in the fluid flow
direction, wherein the flow field heat transfer region includes a
plurality of open enhancement features from about 1 micron to about
3 mm in height; and flowing fluid to the fluid inlet, through the
flow field and out the fluid outlet in a manner to transfer heat in
the heat exchange region.
In an embodiment of the present invention, the heat transfer system
includes a single inlet; a single outlet; multiple fluid inlets;
multiple fluid outlets; or combination thereof, in fluid
communication with the substrate.
The invention provides a flow field incorporating a taper. Taper is
defined as a change in the cross-sectional area of the flow field
in the fluid flow direction. An overall increase from inlet to
outlet of the cross-sectional area of the flow field in the fluid
flow direction is defined as positive taper. An overall decrease
from inlet to outlet of the cross-sectional area of the flow field
in the fluid flow direction is defined as negative taper. The taper
can be uniform, non-uniform, continuous, variable, step-wise, and
combinations thereof. The taper can incorporate regions of
non-taper, positive, negative, or uniform taper, and any
combinations thereof. For example, taper can include a region of
taper, followed by a region or other regions of different taper or
region or other regions of no taper. The taper can be designed to
meet desired thermal characteristics. In an embodiment, the flow
field cross-sectional area is considered tapered when at least some
region of the flow field is tapered in the flow direction.
In accordance with an embodiment of the present invention, the flow
field includes a fluid flow area including an open region at the
inlet, a heat transfer region in thermal communication with the
heat exchange region, and a taper of the flow field cross-sectional
area in the fluid flow direction. In flow boiling, the flow field
incorporates an open region at the inlet and a positive taper for
providing an escape path for vapor in two-phase flow while
promoting liquid flow toward the heat transfer surface during
boiling. The increasing area in the flow direction reduces the
accelerational pressure drop during boiling and aids in reducing
the overall pressure drop. The invention will make a dramatic
impact in heat exchangers, generally and high performance
microscale heat exchangers, such as in the field of electronics
cooling, in particular. In this flow boiling embodiment, the flow
field cross-sectional area is increased in the direction of flow by
providing a positive taper from the fluid inlet to the fluid outlet
and an open region throughout the flow field. In single-phase flow
and in condensation two-phase flow the flow field incorporates an
open region at the inlet and the cross-sectional area is decreased
in the direction of flow by providing a negative taper from the
fluid inlet to the fluid outlet. In single-phase flow and in
condensation two-phase flow the flow field may incorporate a closed
region in the flow field downstream from the open region, for
example, at the outlet. In single-phase and two-phase flow, the
taper of the flow field area can be coupled with other enhancement
techniques, as desired. In these applications, the taper can be
configured as positive, negative, uniform, non-uniform, stepped and
similar regions individually or in any combination to provide
desired heat transfer and pressure drop performance.
The phrase flow field is used to designate a region that fluid
flows through to affect heat transfer in the system. An open region
of the flow field is a region of the flow field containing a gap
above the enhancement features wherein the fluid flow is mixed
throughout the open region. For example, a flow field containing a
plurality of open microchannels throughout is considered a mixed
flow field since the fluid flow is mixed throughout the open
region. For example, a plurality of closed microchannels at the
inlet region of a flow field is considered an unmixed flow field
region since the fluid flow is unmixed throughout the closed
region. The flow field in accordance with the present invention
includes a heat transfer region containing a plurality of
enhancement features and an open region incorporating a gap above
the enhancement features. An individual channel with a heat
transfer region and an open region above the enhancement features
is considered a mixed flow field. The flow field heat transfer
region is a region of the flow field which exchanges heat with heat
transfer surfaces of the substrate. An open enhancement feature is
an enhancement feature having a gap above the enhancement feature
wherein the fluid flow over the heat transfer surface of the
substrate is in communication with the fluid flow in the gap. In an
embodiment, the flow field contains enhancement features,
including, but not limited to, pin fins, microchannels, uniform
roughness and structured roughness elements, turbulators, vortex
generators, non-uniform roughness, projections, pins, micro fins,
nanowires, channels, porous surfaces, microstructures,
nanostructures, turbulators, vortex generators, and the like, and
combinations thereof.
The term "microchannel" is used to indicate microchannels and
minichannels. The microchannel can be straight or wavy, may
incorporate enhancement features, such as uniform or structured
roughness, or secondary features including, but not limited to
nanowires, nanostructures, porous matrix, or other features and
combinations thereof. The microchannel and other features may be
created by any fabrication process, including, but not limited to,
etching, ablating, sintering, machining, stamping, embossing,
electroplating, laser machining, water jet, plastic deformation
technique, and the like. The term microchannel also includes
enhancement features, including, but not limited to, pin fins of
any shapes, offset-strip fins, delta wings and flow
turbulators.
Taper can be effectively implemented, for example, by making fins
shorter or longer along the flow length. To make fabrication
easier, the fin height may be changed in a stepwise fashion.
Alternatively, the cover may be fabricated with a taper resulting
in a change in the flow area. The varying roughness, geometry,
and/or density of enhancement features, such as fins,
two-dimensional and three-dimensional roughness structures can be
incorporated to yield similar thermohydraulic effect as taper.
In accordance with an embodiment of the present invention, the flow
field area is changed without changing the width at the prime
surface of the heat exchange region alone. On a heat exchanger
surface composed of fins, the fins are conceptualized as being
placed on the base surface. The heat transfer area of the base
surface that is in communication with the fluid is referred to as
prime area and the surface area of the fins is referred to as the
fin area. The total heat exchange surface area is the sum of the
prime and fin surface areas. In the case of nanowires,
nanostructures, and some microstructures, the area enhancement of
these features is not considered separately; but reflected in the
enhancement in the heat transfer coefficient.
Changing the flow field cross-sectional area in the flow direction
without substantially changing the width at the prime surface of
the heat exchange region alone can be accomplished according to one
embodiment, by changing the height of the channel without
substantially changing the width of the channel, wherein for
example a channel represents the flow field and the base of the
channel represents the prime surface of the heat transfer region.
So, for example, in a heat exchange apparatus wherein the fluid
flow field is represented by a series of adjacent channels, the
flow field cross-sectional area can be changed by changing the
height of the channels in the desired manner without substantially
changing the width of the substrate that represents the prime
surface of the heat exchange region. Further, the flow field
cross-sectional area can be changed in accordance with another
embodiment by changing the height of the channels in the desired
manner in combination with changing the width of the substrate that
represents the prime surface of the heat exchange region. In other
embodiments this can be accomplished by other manners.
In a heat exchanger composed of multiple parallel microchannels,
the prime surface of the heat exchange region associated with a
channel may be identified as the area associated with the base of
the channel. At any flow cross-section, representing a
cross-section normal to the overall flow direction, the width of
the microchannel at the base is considered to be the prime surface
of the heat exchange region. For a microchannel with constant flow
cross-sectional shape along the flow length, the width of the flow
cross-section at the prime surface remains constant along the flow
length. Changing the flow field cross-sectional area, by changing
the width of the cross-section at the prime area along the flow
direction, while the height and shape of the fins remain
essentially unchanged, is considered changing the width at the
prime surface of the heat exchange region alone.
In contrast, as an example, the following different ways describe
how the flow field cross-sectional area can be changed without
changing the width of the flow cross-section at the prime surface
alone. The change in the flow cross-sectional area along the flow
length can be accomplished by changing the width of the
cross-section in different region or regions other than at the
prime surface. For example, a rectangular flow cross-section may
change to a trapezoidal flow cross-section, while the width of the
flow cross-section at the prime surface may remain constant, as the
width at the other opposite side of the rectangle is changed. The
flow cross-sectional area may alternatively be changed by changing
the flow cross-sectional area through changing the height of the
flow cross-section.
In the case of a pin fin, the flow cross-sectional area may be
changed in the flow direction by changing the fin density along the
flow direction. This represents changing the flow cross-sectional
area by changing the width of the flow cross-section at the prime
surface alone. Adding a gap above the fins to provide a gap whose
flow cross-sectional area (of the gap) changes along the flow
direction provides another way to change the flow cross-sectional
area of the flow field. This arrangement with the gap may be
accomplished with uniform fin density or varying fin density along
the flow length. This arrangement also represents an example of how
the flow cross-sectional area can be changed without changing the
width of the flow cross-section at the prime surface alone.
An embodiment of this invention provides at least three different
methods in which the flow resistance and heat transfer
characteristics can be changed along the flow direction. One way is
to change the flow cross-sectional area in the flow direction,
without changing the width of the flow cross-section at the prime
surface of the heat exchange region alone. A second way is to
change the geometry, density or the size and shape of the surface
and/or enhancement features along the flow direction, without
changing the width of the flow cross-section at the prime surface
alone. A third way is a combination of the previous two ways
described earlier in this paragraph.
As an example of the second method, the flow resistance and the
heat transfer characteristics can be changed by changing the
surface roughness of the surfaces encountered by the fluid along
the flow length. This may be accomplished by changing the pitch of
the roughness structures along the flow length. As another example
of the second method, the density of the flow turbulators placed on
the surfaces encountered by the fluid flow may be changed along the
flow direction.
In accordance with an embodiment of the present invention critical
heat fluxes are obtained above 400 W/cm.sup.2, preferably above 500
W/cm.sup.2, more preferably above 600 W/cm.sup.2, including 260
W/cm.sup.2 at 3 kPa, 350 W/cm.sup.2 at 10 kPa, and 650 W/cm.sup.2
at 6.5 kPa, using water as the fluid.
In accordance with an embodiment of the present invention
enhancement feature height ranges include about 1.mu.-3 mm,
1.mu.-1000.mu., 10.mu.-500.mu., and 160.mu.-400.mu..
In accordance with an embodiment of the present invention pressure
drops of less than about 10 kPa over 10 mm flow length are achieved
for dissipating from about 400-600 W/cm.sup.2 heat flux from a 10
mm.times.10 mm chip surface.
The details and workings of embodiments in accordance with the
invention are schematically shown in the following Figures. FIG. 1
is a schematic representation of a substrate 1, in this example a
microchannel chip system 2 on which the microchannel enhancement
features of the substrate conduit are incorporated. Although the
substrate 1 is shown as planar, it may be of any other shape,
including cylindrical or of any other configuration. The
microchannels 3 may be of any cross-section, including, but not
limited to, rectangular, square, triangular, trapezoidal, and
combinations of these and other possible configurations that
includes the description given in the above paragraphs. These
channels are generally parallel, but may deviate from being
parallel, and the cross-section may be non-uniform along the flow
length. The channels may be along the fluid flow direction or at
any other angle, including 90 degrees. The microchannel region can
also be replaced by one or more other enhancement features such as
pin fins, 2d and 3d roughness features, uniform or structured
roughness features, turbulators, vortex generators, flow modifiers,
nanowires, nanostructures, and the like, and combinations
thereof.
FIG. 1 shows a schematic of a cover block 4 with inlet 5 and outlet
6 ports. These ports serve to provide inlet and outlet passages for
the fluid flow in the open region 7 and flow in the heat transfer
region 20 of the flow field 10. The orientation and the
configuration of the port passages within the cover block may be
different so as to provide the inlet and outlet, or multiple inlets
and/or outlets, from different faces of the cover block. These
ports may be made of more complex flow passages rather than the
straight with uniform cross section as shown in FIG. 1. The ports
may be incorporated elsewhere, including the substrate 1 or the
heat transfer surface. A gasket 8 can be used between the cover
block 4 and the substrate 1 as shown in this embodiment.
FIG. 1 is a schematic representation of an embodiment of the system
showing a microchannel chip system 2 with a negative taper 9 of
flow field 10 over the microchannels 3. The open region 7 is formed
in this example by the gasket 8 and the cover block 4. Fluid is
supplied through the inlet 5 port, travels through the heat
transfer region 20, shown as a plurality of open microchannels 3,
and the tapered flow field open region 7 and is removed through the
outlet 6 in the cover block 4. Heat is transferred between the heat
transfer surface 11 of the substrate 1, in this example the surface
of the open microchannels 3, and the fluid. The tapered flow field
open region 7 may be created by other manners, such as recessing
the cover block over the microchannel region, or changing the
height of the microchannel fins, or the height of other enhancement
features. The gasket 8 in this embodiment provides a seal and a
gap, however these functions could be achieved by other alternative
manners.
FIG. 1 shows one of the embodiments of providing a flow field 10
with open microchannels 3. This configuration may be achieved by
other manners, such as, but not limited to, a cover block with a
recess over the microchannel region, use of an o-ring for sealing
purposes, or joining the microchannel chip and the cover plate with
glue, anodic bonding, mechanical fixing, soldering, welding, or the
like.
In an embodiment surface features embedded on the surface of the
cover plate 4 can form the tapered open region of the flow field 7.
A smoothly varying 2-dimensional profile is an example. The profile
is preferably a smoothly varying sinusoidal profile, although any
2-dimensional or three-dimensional surface features may be
incorporated, including, but not limited to, turbulators, vortex
generators, and other enhancement features and techniques.
The fluid flow is affected by the surface feature in the vicinity
of the open region over the microchannels. The change in the fluid
flow direction disturbs the flow over the microchannel surfaces
causing heat transfer enhancement. The shape of the surface
feature, the difference between the minimum and maximum open region
along the flow direction, the minimum open region, the microchannel
width and depth, and microchannel fin width are parameters that can
be optimized for given operating conditions and a desired
performance. Similarly, the enhancement feature parameters in other
enhancement techniques may be varied for improved performance. For
the 2-dimensional roughness features the ratio of the pitch to the
difference between the minimum and maximum flow field height is a
parameter that can affect the pressure drop and heat transfer
performance.
Roughness features in accordance with the invention include plain
microchannels with fins which generate an enhancement profile on
the sidewalls of the microchannels. The fluid flow through the open
microchannel region encounters varying cross-sectional area along
the flow length. This causes an enhancement in the heat transfer.
Other surface features may be incorporated on the side walls and/or
the bottom walls and the surface of the fin region exposed to the
flow field open region. These surface features may be used alone,
or some combination thereof, or along with the surface features
incorporated on the cover block.
In single-phase flow, a negative taper 9 of the flow field 10
cross-sectional area in the direction of fluid flow can be seen in
FIG. 1, which shows one embodiment of the present invention having
an open region which provides additional fluid flow cross-sectional
area for the fluid flow. This additional area reduces the pressure
drop as compared to the flow field without any open region. Higher
fluid flow rates can be introduced for a given allowable pressure
drop as compared to microchannels not having additional area
provided by the open region. The advantage of additional fluid flow
in the case of single-phase flow is that the temperature rise of
the fluid, due to the heat transfer from the microchannels to the
fluid, is lowered. This allows for larger heat transfer to occur
for a given allowable fluid temperature rise. Conversely, the fluid
temperature rise will be lower for a given heat transfer rate with
the higher fluid flow rate, and thus the maximum wall temperature
of the microchannel surface will also be lower.
For a fluid mass flow rate of m kg/s, the fluid temperature rise in
the case of single-phase flow from the inlet T.sub.f,in to outlet
T.sub.f,out depends on the heat transfer rate to the fluid,
neglecting losses q=mC.sub.p(T.sub.f,out-T.sub.f,in) (1) where
C.sub.p is the specific heat of the fluid, taken at the average
fluid temperature. Increasing m allows for a larger heat removal by
the fluid for the same fluid temperature rise. The corresponding
pressure drop is given by
.DELTA..times..times..times..times..times..times..times..rho..times..time-
s..times. ##EQU00001## where f.sub.ave is the average friction
factor, L is the flow length, .rho. is the fluid density, A.sub.c
is the flow cross-sectional area, and D.sub.h is the hydraulic
diameter of the flow channel. By increasing the flow area and/or
hydraulic diameter, the pressure drop goes down. The friction
factor also depends on the flow rate. For laminar flow, the fully
developed friction factor f.sub.D is given by
.times..times..times..mu..times..times. ##EQU00002## where .mu. is
the viscosity of the fluid. The average friction factor f.sub.ave
is higher than f.sub.D due to entrance region effects. An estimate
of the pressure drop can be obtained by neglecting the entrance
region effects, and combining Equations (2) and (3). It is seen
that an increase in the flow area substantially decreases the
pressure drop.
The heat transfer rate is given by the following heat transfer
equation. q=hA.sub.s(T.sub.s-T.sub.f) (4) where h is the heat
transfer coefficient, A.sub.s is the heat transfer area, and
T.sub.s and T.sub.f are the surface and fluid temperatures. Since
the temperatures vary along the flow length, an integration is
preferred. Nevertheless, it is clear that increasing h and A.sub.s
lead to an increased heat transfer rate. In the case of finned
surfaces, fin efficiency may be included in evaluating the heat
transfer from the fin surfaces.
In the open microchannel geometry having an open region, the heat
transfer coefficient over the heated surface is high due to the
presence of the microchannels or other enhancement features in the
flow field. The presence of an open region provides an extra area
for fluid flow, thereby reducing the pressure drop for a given flow
rate as compared to the flow through the microchannel area (closed
microchannel) only, and offers a very attractive solution for heat
transfer from the surface under a given pressure drop constraint.
In addition, during flow boiling, the acceleration pressure drop
decreases because of the increasing area provided by the positive
taper and greatly reduces the total pressure drop. Surface features
can be incorporated on one or more of the surfaces of the
microchannels and the cover block. These surfaces are provided to
enhance the heat transfer performance through a higher heat
transfer rate for a given temperature difference.
In single-phase flow the flow field cross-sectional area is reduced
in the direction of fluid flow by the negative taper of the flow
field area. This can be implemented in an embodiment which provides
a tapered cover block over the flow field and a reduction of the
flow field cross-sectional area in the direction of fluid flow. The
heat transfer coefficient in a uniform flow field arrangement is
constant along the flow length, barring for the effects due to
fluid property changes with temperature and entrance region
effects. For the case where heat flux is more or less uniform, this
leads to a temperature variation of the substrate along the flow
direction. To account for this, in a preferred embodiment, a
negative tapered 9 flow field 10 is introduced as shown in FIG. 1.
The gap is higher at the fluid entrance and decreases along the
flow length. This reduces the flow field cross-sectional area and
causes the heat transfer coefficient to increase along the flow
length. The wall temperature T.sub.W at any section may be
expressed as a function of the local heat flux q'' and the local
fluid temperature T.sub.f
'' ##EQU00003## since T.sub.f increases along the flow length, an
increase in h will be able to offset by reducing the second term in
Equation (5). This is accomplished by providing the negative taper
as shown in FIG. 1. The flow field cross-sectional area reduces
from inlet 5 to outlet 6, thereby increasing the heat transfer
coefficient. The increase in pressure drop due to the positive
taper 12 is adjusted by changing the taper angle or the effective
taper variation and the initial gap at the inlet section.
Similar effect can be accomplished by another embodiment having a
roughness feature on the cover block with an increasing roughness
along the flow length on the cover block, effectively resulting in
a taper of the flow field area. As the roughness increases, the
heat transfer coefficient increases along the flow length and the
wall temperature will tend to be more uniform along the flow length
as given by Equation 1. The change in the roughness or other
feature density provides a similar effect as a taper.
There are other techniques that can be implemented to provide an
increasing h along the flow direction, including, but not limited
to, increasing density of surface features, such as microchannels,
fins, turbulators and flow modifiers along the flow length, which
are effectively another form of taper.
FIG. 1 shows an embodiment in which the flow field cross-sectional
area along the flow length varies continuously due to continuously
varying negative taper 9 in the height of the microchannel fins
forming the channel walls, although other techniques could be
implemented to achieve a similar effect. Flow channels are created
by covering the top surface of the microchannel chip with a cover,
although different configurations could be implemented to achieve a
similar effect. Further, this continuous varying of the taper can
be implemented in a positive taper 12 in the cross-sectional area
along the flow direction as shown in FIG. 2.
During flow boiling in microchannels there are four main issues
that are encountered. (1) High pressure drop, (2) Flow instability,
(3) Low Critical Heat Flux (CHF), and (4) Low heat transfer
coefficient. A microchannel configuration embodiment with cover
block 4 employing flow boiling is shown in FIG. 2. The flow field
10 open region 7 provides the extra space needed as the liquid
evaporates into vapor, whose specific volume is higher than the
liquid. The increase in the flow area reduces the pressure drop as
compared to the flow in microchannels without the open region. The
pressure drop reduction also comes from the reduction in the
acceleration pressure drop due to increasing area along the flow
direction. The arrangement reduces the severity of the flow
instabilities. The liquid flows preferentially through the
microchannels due to higher resistance to vapor flow, and/or
capillary and/or gravitational forces and supplies the liquid over
the heat transfer surfaces. The vapor flows preferentially through
the gap. The separate pathways provided in the microchannels with
open flow field configuration helps in increasing the CHF. The flow
configuration also helps in improving the heat transfer
coefficient.
FIG. 2 shows flow boiling with a positive tapered flow field
configuration. The flow field cross-sectional area is tapered from
the fluid outlet to the fluid inlet. The flow area increases in the
fluid flow direction. This configuration reduces the backflow
caused during the boiling process, and improves the flow stability.
There may be other types of instability present in the system, such
as those due to the inlet volume compressibility effects, and a
valve may be used prior to the inlet to avoid those types of
instabilities. Since the flow area is getting larger in the flow
direction, the pressure drop is smaller for a given flow rate and
for a given heat removal rate as compared to flow inside closed
microchannels. The heat transfer performance of the tapered flow
field embodiment is improved and pressure drop is reduced as
compared to an arrangement utilizing a uniform gap having the gap
set as the minimum gap in the tapered gap embodiment.
In another embodiment the flow cross-sectional area along the flow
length varies in a step-wise or non-continuous fashion due to
step-wise varying taper in the heights of the microchannel fins
forming the channel walls, although different configurations could
be implemented to achieve a similar effect. Flow channels are
created by covering the top surface of the open microchannels and
the gap with a cover, although different configurations could be
implemented to achieve a similar effect. The step-wise varying
taper could be reversed to realize a reverse taper in the
cross-sectional area along the flow direction.
FIG. 3 shows the positive taper 30 differing in several regions of
the flow field 31. It is possible to have positive taper or
negative taper or a region with uniform gap in different regions in
any combination with other regions of taper. Further, a tapered
flow field includes a flow field where there would be at least one
region where there is taper.
A flow field can incorporate separate or interconnected flow
passages through which a fluid medium flows while exchanging heat
with the heat transfer surfaces. FIG. 4 shows flow areas associated
with an embodiment in accordance with this invention. In the
present invention, the flow cross-sectional area changes due to
taper. A.sub.FF,HTR 41 represents the flow cross-sectional area in
the flow field in the heat transfer region. Due to the presence of
gap, the additional flow cross-sectional area at any cross-section
is A.sub.FF,Gap 42. The total flow cross-sectional area at any
cross-section is A.sub.FF,HTR plus A.sub.FF,Gap. Preferred
dimensions in accordance with an embodiment of the invention are as
follows. The ratio of the maximum fluid flow cross-sectional area
to the minimum fluid flow cross-sectional area,
(A.sub.FF,Gap+A.sub.FF,HTR)/A.sub.FF,HTR is preferably in the range
of from about 1.0001 to about 1000, more preferably from about
1.001 to about 100, and most preferably from about 1.01 to about
10.
The heated perimeter may vary along the flow length due to the
taper. The minimum heated perimeter is P.sub.min and the maximum
heated perimeter is P.sub.max.
In accordance with one embodiment of the present invention, the
pressure drop in the heat transfer system of the flow field is
allowed to be reduced due to the introduction of the cover gap.
Alternatively, for a given pressure drop limit, the fluid flow rate
through the heat transfer system is increased. The heat transfer
performance in the system can be affected in two ways. The heat
transfer coefficient may be reduced due to the addition of
increased flow area in the cover gap. This decrease is more than
offset by the heat transfer performance, as measured by the total
heat transfer rate, or in terms of heat transfer rate for a given
pressure drop limit, due to the reduction in the pressure drop
caused by the addition of the increased flow area in the cover
gap.
As the flow cross-sectional area decreases due to the negative
taper, the heat transfer coefficient may increase in the case of
the single-phase flow at the given location. By adjusting the
change in the heat transfer coefficient coupled with the heat
transfer area changes and the fluid temperature variation in the
flow direction, the substrate temperature could be controlled to
remain substantially uniform, or to change in a certain desired
fashion. The design results in a lower pressure drop as compared to
a heat exchanger without a taper that is designed to meet the same
substrate temperature limit.
As the flow cross-sectional area increases due to the positive
taper, the heat transfer coefficient may increase in the case of
the flow boiling at the given location, the flow becomes stable and
the pressure drop reduces as compared to the configuration having a
uniform cross-sectional area set at the minimum area in the case of
the positive taper configuration. In the case where the
cross-sectional area is maintained uniform at the highest value in
the case of the positive tapered configuration, although the
pressure drop is lower in the case of the uniform cross-sectional
channel, the heat transfer performance is also lower and/or the
flow is less stable. The actual taper and the flow cross-sectional
area can be adjusted to yield stable operation and the desired heat
transfer performance for a given pressure drop limit.
FIG. 5A shows another embodiment of the present invention. The
substrate 51 is composed of a radial fin region 52 representing the
heat transfer region. This region is shown to be circular, but it
can take any other shape, such as square, triangular, oval and
like. A cover 53 is placed above the fin region 52 with a gap 54
between the top of the fin surface and the cover 53. The gap 54 has
a negative taper in the radial direction. The taper may be
continuous or in a step-wise fashion. The cover 53 is shown to be
circular, but it can take any other shape, such as square,
triangular, oval and like. Additional features are present to
contain and direct the fluid flow from inlet 55 to outlet 57 in the
desired fashion. The cover and the fin region may be of the same
size and shape, or may be of different size or shape.
FIG. 5B shows a top view of the microchannel chip alone. The radial
fin region 52 is also shown. The fin region 52 may be composed of
radial open microchannels, with varying channel widths, or with
varying fin widths, or a combination of the two. Alternatively, the
fin region may contain other enhancement features such as pin fins,
offset strip fins, turbulators, structured roughness elements,
microchannels, and like, or combinations of some of these elements.
The taper in the cover is designed to provide the desired
performance in association with the varying flow field
cross-sectional area in the flow direction within the fin region
from inlet 55 to outlet 57. The orientation of the enhancement
features may be along the flow direction or parallel to one of the
edges, or at an angle to the edges or the flow direction, random,
or a combination of any of these. The enhancement features may be
of uniform density, or the density may vary.
The inlet port 55 is shown in the center in FIGS. 5A and 6A. The
inlet port may be located off-center, or at any other location.
Multiple ports may be provided instead of a single port. The taper
may be uniform, non-uniform or step-wise. The outlet 57 is shown to
be uniform all around. It may be open only at certain specific
locations. The inlet and outlet ports may be switched. The
configuration may be used for single-phase, or two-phase, or
combination, including flow boiling and flow condensation.
FIGS. 6A and 6B show another embodiment in which the inlet port 65
is rectangular and the fluid flows in the two opposite directions.
The number of the ports, shape of the ports, and their locations
can be same or different from those shown in these embodiments. The
size and shape of the cover and the fin region may be the same or
different from each other. These shapes can be other than those
shown in these embodiments. In a further embodiment, the cover may
include additional surface enhancement features, such as those
noted above.
The invention can be used with single-phase fluid flow or two-phase
fluid flow, or both in different regions, and includes flow boiling
and flow condensation. Additional features may be incorporated to
improve the heat transfer rate, reduce pressure drop, provide flow
stability, remove or add one phase preferentially from or to the
heat transfer system. Specific features are further elaborated in
the description provided in the disclosure.
The invention will be further illustrated with reference to the
following specific examples. It is understood that these examples
are given by way of illustration and are not meant to limit the
disclosure or the claims to follow.
The flow boiling experiments were performed using the configuration
shown in FIG. 7, which shows open microchannels with a tapered gap
that provides larger flow area toward the outlet for stable,
enhanced flow boiling. FIG. 8 shows a schematic of the test section
and heater assembly used in the examples which is composed of a
copper block with eight 200 W cartridge heaters to serve as the
main heating unit. The tip of the square heating block measures 10
mm.times.10 mm and has three equally spaced K-type thermocouples
that were used to determine the heat flux. A copper chip contacts
the tip of the heating block, and a fourth thermocouple in the chip
measures its temperature. The chip was supported by a ceramic
plate, and a cover block delivers and removes water from the
surface of the chip. A silicone gasket between the chip and the
cover block was used to seal the system and provided the ability to
readily vary the gap by changing the gasket thickness. The gaskets
used during testing had thicknesses ranging from 0.127 mm to 1.524
mm. A Keyence VW-6000 camera with recording speeds of up to 24,000
frames per second (fps) was used for recording and observing flow
boiling patterns, though most of the videos were recorded at
1000-2000 fps. A pump provided varying flow rates of distilled
water to the test setup. A reservoir containing the water (not
shown) was heated to saturation temperature with an electric hot
plate. To overcome the heat losses in the supply line between the
reservoir and the test section, an inline heater was installed and
a subcooling of 2-5.degree. C. was maintained at the inlet of the
test section. The supply tubing, pump, and flow meter were wrapped
in fiberglass insulation to minimize heat losses.
Prior to conducting tests, the gasket, test chip, cover block, and
water flow rate were selected, and the heater was initialized at 35
V input (corresponding to a minimum heat flux of about 24
W/cm.sup.2). Once the system achieved steady state (characterized
by a negligible change in heat flux over a 10 minute period) the
thermocouple temperatures and pressure drop readings were recorded.
The heater voltage was then increased by 5 or 10 V, and the system
was again allowed to reach a new steady state. For safety purposes
this testing cycle continued only until the chip surface
temperature approached 125.degree. C., or the cartridge heater
temperature reached 600.degree. C. High-speed videos of the chip
surface were taken at each set point for visualization of water
flow and bubble dynamics. Following the completion of a test, the
power supply to the heater was shut off so that the setup can cool
down. The water flow rate was then adjusted to the next set point,
and the tests were repeated. A maximum of five flow rates were
tested during the experiments.
The heat flux to the chip was calculated by the Fourier's law for
one dimensional heat conduction in the heater block:
''.times.dd ##EQU00004##
The thermal conductivity of the copper heater block is k.sub.Cu.
The temperature gradient dT/dx was calculated from a three-point
backward difference Taylor series approximation using the heating
block temperatures, T.sub.1, T.sub.2 and T.sub.3, each at a
distance .DELTA.x of 8 mm apart:
dd.times..times..times..times..times..DELTA..times..times.
##EQU00005##
Finally, the chip surface temperature was obtained from the
measured chip temperature T.sub.4, the heat flux, and the distance
L=1.5 mm between the chip thermocouple and the chip surface.
''.times. ##EQU00006##
An uncertainty analysis was conducted, at the highest flow rate
(333 mL/min) the rotameter had an uncertainty of 3%, and at the low
flow rate the uncertainty was 5%. The individually calibrated
thermocouples have an accuracy of 0.1.degree. C. The resulting
uncertainty in heat flux and surface temperature at high heat
fluxes were 4% and 0.24.degree. C., respectively. All heat fluxes
are reported on the basis of the projected area of the boiling
surface, which is 100 mm.sup.2.
Test Chips--Experiments were performed with two 3 mm thick copper
chips: one with a plain surface, and the other with microchannels
in the central 10 mm.times.10 mm heated region. The front and back
views of the microchannel chip are shown in FIG. 9. The open
microchannels were CNC milled and had a channel width of 217 .mu.m,
fin width of 160 .mu.m, channel depth of 162 .mu.m and length of 10
mm in the central 10 mm.times.10 mm region (left image) and 2 mm
wide.times.2 mm deep groove on the underside (right image).
Specifically, a 2 mm wide and 2 mm deep groove was machined on the
underside of the chip to reduce the heat losses and heat spreading
effect from the chip.
The cover blocks were constructed out of Lexan.RTM. and
polysulfone, and were polished to improve transparency for flow
visualization with the high speed camera. Two types of cover blocks
were employed: a positive tapered cover block, which provided a
tapered gap above the microchannels and increased the gap depth by
0.180 mm from the inlet to the outlet, and a uniform cover with no
taper which had a flat surface without any recess in the block. In
both cases the depth was controlled by inserting a gasket of
desired thickness between the chip and the cover block as shown in
FIG. 8. Five gaskets of thicknesses 0.127 mm, 0.254 mm, 0.508 mm,
1.016 mm, and 1.524 mm were used to provide the desired gap depth
over the microchannels.
The gaskets served a secondary purpose of limiting heat transfer to
the microchannel region. A 10 mm.times.10 mm opening in each gasket
was aligned over the 10 mm.times.10 mm microchannel region to
inhibit contact between the working fluid and the outer edges of
the test chip. The heat losses and the heat spreading effect from
the test section were minimized due to the gasket and the groove on
the underside as shown in FIG. 9. The errors in heat flux due to
heat losses and heat spreading effects were estimated to be less
than 1%. The active area of the chip was thus defined by the square
opening in the gasket over the chip region with microchannel
features. The microchannel flow length was 10 mm in all
testing.
NOMENCLATURE
As used throughout this application, the following symbols are
meant to represent:
h heat transfer coefficient, W/m.sup.2.degree. C.
k.sub.Cu thermal conductivity of copper block, W/m.degree. C.
L length from thermocouple to chip surface, mm
q'' applied heat flux, W/cm.sup.2
P pressure, Pa
V volumetric flow rate, mL/min
S microchannel depth (gap), mm
T temperature, .degree. C.
x vapor quality
Greek Symbols
.DELTA.T.sub.sat wall superheat, .degree. C.
.DELTA.x distance between thermocouples, mm
Superscripts
s surface
EXAMPLES
The experimental results given below report the heat flux values,
pressure drop, and heat transfer coefficients obtained in each of
the following experiments.
Example 1
This example is reported in a paper entitled "Experimental
Investigation of Flow Boiling Performance of Open Microchannels
with Uniform and Tapered Manifolds (OMM)" by Kandlikar, Wedger,
Kalani and Mejia published in ASME Journal of Heat Transfer, Volume
135, June 2013, which is herein incorporated by reference in its
entirety. A microchannel chip with a 10 mm.times.10 mm area was
used as the boiling surface. The open microchannels were 162 .mu.m
(1 .mu.m=0.001 mm) deep and 217 .mu.m wide with 160 .mu.m wide fins
separating the open microchannels. Two different configurations A
and B with different gap spacings S above the open microchannels at
the inlet of (A) 127 .mu.m and (B) 254 .mu.m were tested. The
taper, which represents the difference between the outlet gap and
the inlet gap, was 180 .mu.m for both gaps. The resulting gaps
above the open microchannels at the outlet were thus (A)
127+180=307 .mu.m and (B) 254+180=434 .mu.m, respectively. The
testing was performed with water as the fluid at two different flow
rates V of (1) 40 mL/min (milliliters per minute) and (2) 225
mL/min. The resulting heat transfer performance is shown in FIG. 10
with heat flux on the projected heat transfer area of 10
mm.times.10 mm plotted as a function of wall superheat (which is
the wall surface temperature minus the saturation temperature of
the boiling liquid) for the two gaps and the two flow rates.
In all the experimental runs, the critical heat flux was not
reached. Experiment A1 (V=40 mL/min, S=0.127 mm) a maximum heat
flux of 350 W/cm.sup.2 at a wall superheat of 19.degree. C. was
obtained with an inlet gap above the open microchannels of 127
.mu.m at 40 mL/min. The resulting heat transfer coefficient
(defined as the heat flux divided by the wall superheat) was
184,200 W/m.sup.2.degree. C. This performance is significantly
above the heat transfer performance obtained with closed
microchannels of similar dimensions and flow rates, for which
literature indicates a maximum heat flux in the neighborhood of 130
W/cm.sup.2 with a heat transfer coefficient of 20,000 to 80,000
W/m.sup.2.degree. C. The reduction in pressure drop was similarly
very significant. For prior art configurations using closed
microchannels of similar dimensions and flow rates a pressure drop
of 40-80 kPa at heat fluxes of less than 130 W/cm.sup.2 has been
reported in the literature. For Experiment A1, the flow was found
to be stable with no back flow seen during high speed visualization
through a transparent cover. In Experiments A1 (V=40 mL/min,
S=0.127 mm), A2 (V=225 mL/min, S=0.127 mm), B1 (V=40 mL/min,
S=0.254 mm), B2 (V=225 mL/min, S=0.254 mm), the critical heat flux
was not reached indicating potential for higher heat flux
dissipation with the tapered open microchannel configuration of the
present invention. These results indicate significant improvement
in all three parameters with higher maximum heat flux dissipated,
higher heat transfer coefficient and lower pressure drop as
compared to the boiling systems reported in the literature.
Example 2
These tests were performed on two chips--a plain chip without any
microchannels and a microchannel chip similar to that used in
Example 1 except with microchannels 450 .mu.m deep and 181 .mu.m
wide, and with 195 .mu.m wide fins separating the microchannels.
The projected surface area of the heated chip over which boiling
occurred was the same in all three examples, i.e., 10 mm.times.10
mm.
Experiment U2 had a uniform gap of 127 .mu.m over the entire chip
area from inlet to outlet. Three different tapered gaps of 127
.mu.m at the inlet and 327 .mu.m, 527 .mu.m and 727 .mu.m at the
outlet (127 .mu.m gap plus 200 .mu.m, 400 .mu.m and 600 .mu.m
taper, respectively) were used, as represented by C1, C2, and C3,
respectively each with gradually increasing gap on the exit side.
The mass flux for the testing was kept constant at 1050 kg/m.sup.2s
based on the flow area at the inlet for all the experiments in
Example 2.
FIG. 11 shows the heat transfer performance comparison of the
uniform gap of U2 and the three taper configurations of C1, C2, and
C3. The taper C3 configuration provided the best performance in
this group with a heat flux of 281.2 W/cm.sup.2 at a wall superheat
of 10.1.degree. C. The resulting heat transfer coefficient was
278,400 W/m.sup.2.degree. C., which is higher than any value
reported for flow boiling in closed microchannels in the
literature. In addition C3 provided the best pressure drop
performance in this group. FIG. 12 shows pressure drop comparisons
for these four configurations. It is seen that all tapered
configurations performed significantly better than the uniform gap
configuration. A pressure drop of only 3.3 kPa was obtained with
taper C3. The CHF was not reached in any of the tests shown in
FIGS. 11 and 12. This combination of high heat flux dissipation,
high heat transfer coefficient, and low pressure drop shows the
unexpected behavior for configurations employed in these tests.
Example 3
The third example describes the performance of a microchannel chip
of the same overall projected heating surface area of 10
mm.times.10 mm as in Examples 1 and 2. The microchannel chip had
microchannels which were 250 .mu.m deep with a channel width of 205
.mu.m and a fin width of 145 .mu.m. The taper was 340 .mu.m with an
inlet gap of 180 .mu.m at the inlet resulting in an outlet gap of
520 .mu.m.
FIG. 13 shows the flow boiling performance of experiment D with a
water flow rate of 80 mL/min. This configuration provided a heat
flux of 648.5 W/cm.sup.2 at a wall superheat of 13.04.degree. C.,
resulting in a heat transfer coefficient of 497,300
W/m.sup.2.degree. C., which is a record for any comparative flow
boiling systems reported in literature. The corresponding pressure
drop plot is shown in FIG. 14, indicating at the highest flux, the
resulting pressure drop was 6.41 kPa.
Although various embodiments have been depicted and described in
detail herein, it will be apparent to those skilled in the relevant
art that various modifications, additions, substitutions, and the
like can be made without departing from the spirit of the invention
and these are therefore considered to be within the scope of the
invention as defined in the claims which follow.
* * * * *
References